Commit 12e967fd authored Mar 21, 2020 by Michal Hocko Committed by Linus Torvalds Mar 21, 2020

mm: do not allow MADV_PAGEOUT for CoW pages

Jann has brought up a very interesting point [1].  While shared pages
are excluded from MADV_PAGEOUT normally, CoW pages can be easily
reclaimed that way.  This can lead to all sorts of hard to debug
problems.  E.g.  performance problems outlined by Daniel [2].

There are runtime environments where there is a substantial memory
shared among security domains via CoW memory and a easy to reclaim way
of that memory, which MADV_{COLD,PAGEOUT} offers, can lead to either
performance degradation in for the parent process which might be more
privileged or even open side channel attacks.

The feasibility of the latter is not really clear to me TBH but there is
no real reason for exposure at this stage.  It seems there is no real
use case to depend on reclaiming CoW memory via madvise at this stage so
it is much easier to simply disallow it and this is what this patch
does.  Put it simply MADV_{PAGEOUT,COLD} can operate only on the
exclusively owned memory which is a straightforward semantic.

[1] http://lkml.kernel.org/r/CAG48ez0G3JkMq61gUmyQAaCq=_TwHbi1XKzWRooxZkv08PQKuw@mail.gmail.com
[2] http://lkml.kernel.org/r/CAKOZueua_v8jHCpmEtTB6f3i9e2YnmX4mqdYVWhV4E=Z-n+zRQ@mail.gmail.com



Fixes: 9c276cc6 ("mm: introduce MADV_COLD")
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Daniel Colascione <dancol@google.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: "Joel Fernandes (Google)" <joel@joelfernandes.org>
Cc: <stable@vger.kernel.org>
Link: http://lkml.kernel.org/r/20200312082248.GS23944@dhcp22.suse.cz


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent e26733e0

mm/madvise.c

+9 −3

Original line number	Diff line number	Diff line
		@@ -335,12 +335,14 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
		}

		page = pmd_page(orig_pmd);
		if (next - addr != HPAGE_PMD_SIZE) {
		int err;

		/* Do not interfere with other mappings of this page */
		if (page_mapcount(page) != 1)
		goto huge_unlock;

		if (next - addr != HPAGE_PMD_SIZE) {
		int err;

		get_page(page);
		spin_unlock(ptl);
		lock_page(page);
		@@ -426,6 +428,10 @@ regular_page:
		continue;
		}

		/* Do not interfere with other mappings of this page */
		if (page_mapcount(page) != 1)
		continue;

		VM_BUG_ON_PAGE(PageTransCompound(page), page);

		if (pte_young(ptent)) {

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